I want to predict teacher judgment accuracy from predictors at student (dummy coded) and at classroom level (continuous) and possible interactions. I am thus estimating an intercepts and slopes as outcome model.

Also, I have a variable indicating how well the teacher knows a child. As teachers differ in how well they know the kids of their class (for various reasons), my approach was to group-center it and to integrate it at the individual level: [...skip usevar ...] cluster = classno;

but instead you have to define a random slope for each of those 6 predictors separately. You don't have to define random slopes for all of them - in fact, I would explore the need for it for each at a time.